1. Field of the Invention
The present invention relates to thermoplastic bags which are provided with integral handle members on opposite sides of the bag mouth. The structure of the bag handles is especially designed to avoid failure in the handle area when such bags are in use by providing an additional thermoplastic layer at least in the handle area of such bags.
2. Description of the Prior Art
In the past, bags which were characterized by having carrying handles thereon were constructed using separate handle elements, distinct from the bag structure itself. Such handle elements were individually fed, during bag manufacture, for attachment adjacent to the open mouth portion of the bag. The manufacturing operation to produce such prior art structures with the separate process step of supplying handle elements, and applying them to the bag is quite cumbersome and uneconomical. More recently, however, bag structures have been developed, see for example, U.S. Pat. Nos. 4,085,822; 3,352,411; and 3,180,557; and Belgian Pat. No. 862,069, the disclosures of which are incorporated herein by reference, wherein bags are formed so that the handle carrying elements are formed as an integral part of the bag structure itself. That is, the handles are actually an extension of the bag walls. An example of such a bag structure is one that is constructed from a flattened tube or a flattened side edge gusseted tube which is sealed at both ends. A flattened end portion of such a tube is cut off to form an open mouth bag. Conversely, such a bag may be formed by folding a piece of the thermoplastic material on itself, the bottom fold line constituting the bottom part of the bag and heat sealing the upper edge and side wall parts of the bag together. Next a U shaped cutout is made in the upper portion of the bag to provide an opening or entrance, for the introduction of goods to be packaged. The opposite edtes of the upper portion of the bag structure immediately adjacent to the cutout area form loops which may be used to carry such bag structures when they are loaded. In the case of a gusseted tube such handle loops are reinforced, i.e., double ply thickness, by virtue of the presence of the re-entrant or gusset fold in the loop handle members.
Such afore-described prior art bag structures present difficulties for the end user particularly since, for reasons of economy, such structures are usually produced from extremely thin thermoplastic material, i.e., on the order of about 1.0 mil. or less. There is a tendency for such bags to fail in the handle area where, when the loaded bags are being carried, there is a tendency for the stress forces to concentrate. Prior art bag structures of the type hereinabove described are usually formed from low-density polyethylene material.
It has been found that the use of a mono-layer homogeneous composition for both the handle and the body portion of the hereinabove described bags requires necessarily that the resin and thickness choice fulfill the handle tensile strength requirements and body tear and puncture requirements simultaneously. Obviously, such an arrangement is not the most economical utilization of available resin materials. Generally speaking, a single specialty resin which would accomplish all three of these requirements when produced in the form of a relatively thin film structure would be economically prohibitive in the case of the manufacture of the present disposable bag structures.
In general, low density polyethylene has certain physical characteristics which make its employment as a body film desirable for the handle bag constructions of the present invention. In particular, such advantages include its low cost, high puncture resistance, and its resistance to splitting and tearing while under load stress. Conversely, however, as hereinabove noted, low density polyethylene exhibits unsatisfactorily low load bearing ability in the handle regions of such handle bag constructions. The relative low tensile yield properties of low density polyethylene account in part for this deficiency. Accordingly, it is an object of the present invention to improve such bag structures by increasing the tensile yield values in the handle regions of such bags when they are under load.
These and other objects of the present invention may be accomplished by laminating to the handle area of such bags, a film which is specifically designed to improve the overall tensile yield and strength characteristics of the bag in this area. Such film materials include fractional melt index low-density polyethylene, other thermoplastic materials such as linear low-density polyethylene, polypropylene, high-density polyethylene, and thermoplastic films which are highly oriented in the machine direction, including film materials which have been cold drawn, i.e., stretched at ambient temperatures. In addition to cold drawing techniques to achieve the requisite strength characteristics for the reinforcing handle film, such films may also be melted oriented during extrusion. In the case of high-density polyethylene, such melt orientation may be achieved during tubular extrusion thereof by employing low blow up ratios on the order of about 2:1. A range of such low blow up ratios which may be employed to achieve melt orientation of high density polyethylene encompasses from about 1:1 up to about 2.5:1. It has been found that the employment of oriented handle laminating materials in the bag handle area result in improvements in both tensile strength and the yield strength of the film.